acos, acosf, acosl - arc cosine function

```
#include <math.h>
double acos(double x);
float acosf(float x);
long double acosl(long double x);
```

Link with `-lm`

.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

These functions calculate the arc cosine of `x`

; that is the value whose cosine is `x`

.

On success, these functions return the arc cosine of `x`

in radians; the return value is in the range [0, pi].

If `x`

is a NaN, a NaN is returned.

If `x`

is +1, +0 is returned.

If `x`

is positive infinity or negative infinity, a domain error occurs, and a NaN is returned.

If `x`

is outside the range [-1, 1], a domain error occurs, and a NaN is returned.

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

- Domain error:
`x`

is outside the range [-1, 1] `errno`

is set to**EDOM**. An invalid floating-point exception (**FE_INVALID**) is raised.

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |

acos(), acosf(), acosl() | Thread safety | MT-Safe |

C99, POSIX.1-2001, POSIX.1-2008.

The variant returning `double`

also conforms to SVr4, 4.3BSD, C89.

This page is part of release 4.15 of the Linux `man-pages`

project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.